Percarboxylic acid, such as peracetic acid and perpropionic acid, are commonly prepared by feeding hydrogen peroxide and acetic or propionic acid continuously to a reaction medium containing hydrogen peroxide, said carboxylic acid and an acid catalyst, wherein the hydrogen peroxide and the acetic acid or propionic acid react and form percarboxylic acid, and by distilling percarboxylic acid concentrated distillate continuously from the reaction medium.
Percarboxylic acids such as peracetic acid and perpropionic acid can be used as bleaching and disinfecting chemicals. Being environmentally friendly, distilled peracetic acid is very suitable as a chemical for the bleaching of chemical pulp. By using peracetic acid, the total selectivity of TCF bleaching can be improved by carrying out acid delignification stages between the alkaline stages. In addition to delignification, the peracetic acid stages also improves the brightness of the pulp, and therefore, it is well suited for the after-bleaching of ECF and TCF pulps.
The disinfective properties of peracetic acid are utilized e.g. for improving the preservation of pigment suspensions, which enables the recirculation of the waste paste in paper mills. The bleaching properties of the peracetic and perpropionic acids can also be utilized for bleaching the pigment suspension used at the paper machine.
In pulp mills and in the bleaching of pigments, large quantities of peracetic acid are needed both in production and full scale trial runs. In the storage and transportation of distilled peracetic acid, the characteristic features of the chemical must be observed. In order to avoid the characteristic decomposition of peroxides (H2O2=>H2O+½O2), the product is usually stabilized in connection with the preparation. Furthermore, the storage and transportation of peracetic acid must take place under refrigeration (about −10° C.). At elevated temperatures, the peracetic acid is converted back into its starting materials, whereby an equilibrium mixture of the peracetic acid is formed (CH3COOOH+H2O<=>CH3COOH+H2O2).
It is generally known to prepare peracetic acid by feeding hydrogen peroxide and acetic acid continuously to an aqueous reaction medium containing hydrogen peroxide, acetic acid, peracetic acid and acid catalyst, in which medium the hydrogen peroxide and the acetic acid react and form peracetic acid, and distilling peracetic acid concentrate continuously from the reaction medium (U.S. Pat. No. 3,264,346, GB 949,094, GB 1,014,361, EP 296 324, EP 789 016 and EP 1 004 576). As peracetic product, distilled peracetic acid and used reaction medium are obtained.
In these known methods, the bottom product, i.e. the used reaction medium, has only been mentioned as a problematic effluent. A process, in which the bottom product is useful as a disinfectant, odour control agent and microbicide, due to an improvement of its composition, resulting in lower sulfuric acid and metal concentrations, is also known (FI 20010706). On the other hand, it has also been shown, that the bottom product and the distilled peracetic acid concentrate can be combined into a new product, if the acid catalyst of the bottom product is neutralized before the mixing (FI 20010705).
WO-A1-94/20424, WO-A1-01/10215, EP-A1-641777 and DE-A1-3638552 all disclose the preparation of percarboxylic acids in situ for disinfective purposes. As was said in EP-A1-641777, page 3, lines 7 to 11, these processes based on equilibrium mixtures containing peracetic acid, which processes are too slow to produce in situ the required amount of percarboxylic acid.
The transportation of peracetic acid is risky and hence temperature control is required. The transportation to distant sites of use would need the use of refrigerated transport equipment. Besides, refrigerated storage has to be used both on the site of production and the site of use. Thus, both refrigerated transport and refrigerated double storage constitute the main problem lying behind the present invention.